U.S. patent number 3,937,905 [Application Number 05/382,157] was granted by the patent office on 1976-02-10 for moving voice coil transducer having a flat diaphragm of an impregnated knit.
Invention is credited to Josef W. Manger.
United States Patent |
3,937,905 |
Manger |
February 10, 1976 |
Moving voice coil transducer having a flat diaphragm of an
impregnated knit
Abstract
An electro-acoustic transducer has a diaphragm with a moving
coil. The diaphragm consists of a flat textile carrier impregnated
with a highly attenuating filling material and is highly elastic in
its plane but inelastic in bending.
Inventors: |
Manger; Josef W. (8725
Arnstein, DT) |
Family
ID: |
5851586 |
Appl.
No.: |
05/382,157 |
Filed: |
July 24, 1973 |
Foreign Application Priority Data
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Jul 25, 1972 [DT] |
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2236374 |
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Current U.S.
Class: |
381/421; 181/167;
381/401; 381/423; 181/161 |
Current CPC
Class: |
H04R
7/04 (20130101); H04R 7/16 (20130101); H04R
9/063 (20130101); H04R 9/025 (20130101) |
Current International
Class: |
H04R
9/00 (20060101); H04R 7/00 (20060101); H04R
7/16 (20060101); H04R 7/04 (20060101); H04R
9/06 (20060101); H04R 007/04 (); H04R 009/02 () |
Field of
Search: |
;179/115R,115.5R,117,119R,138R,181R
;181/157,161,162,167,169,173 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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70,808 |
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Apr 1950 |
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DK |
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406,749 |
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Mar 1934 |
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UK |
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1,617 |
|
May 1926 |
|
AU |
|
521,988 |
|
Mar 1931 |
|
DD |
|
Primary Examiner: Claffy; Kathleen H.
Assistant Examiner: Stellar; George G.
Claims
I claim:
1. An electro-acoustic transducer system comprising:
a magnetic system having pole shoes and an air gap limited by said
pole shoes;
a substantially flat viscoelastic diaphragm which is highly
elastically stretchable;
fixed support means fixed to said magnetic system and supporting
said diaphragm; and
moving coil means fixed to a portion of said diaphragm.
2. A transducer system according to claim 1 wherein the carrier
element is composed of a fibre material with interlooped
fibres.
3. An electro-acoustic transducer system as claimed in claim 1,
wherein the weight-ratio of said textile carrier element to said
filling material is of the order of 1 to 5.
4. A transducer system according to claim 1 wherein the outer
margin of the diaphragm is fitted to an outer ring which in turn is
fixed to a stationary part of the transducer system.
5. A transducer system according to claim 1 wherein the knitted
material is a warp knitted fabric.
6. A transducer system according to claim 1 the filling material
being applied to the diaphragm under a little pre-tension of the
textile element.
7. A transducer system according to claim 1 wherein the rest
position of the moving coil(s) in its axial direction is wholly
determined by electro-magnetic forces.
8. A transducer system according to claim 1 wherein the diaphragm
is stationarily fixed at its centre.
9. A transducer system according to claim 1 wherein the diaphragm
is fitted to a moving coil former which is coupled to the moving
coil, and wherein the diaphragm is divided by the moving coil
former into sections of substantially the same size.
10. A transducer system according to claim 9 wherein the diaphragm
is of a circular shape and wherein the diameter of the moving coil
former is substantially equal to the radius of the diaphragm.
11. A transducer system according to claim 1 wherein no enclosed
air chambers are provided behind the diaphragm.
12. A transducer system according to claim 11 wherein the moving
coil(s) is/are arranged within an air gap the inner marginal edge
of which is limited by a thin pole shoe ring.
13. A transducer system according to claim 11 including permanent
magnets arranged in a plurality of segments at a given spacing
around a circular arc.
Description
Electro-acoustic transducer systems have been devised for the
conversion of applied electrical oscillations into mechanical
vibrations which are consequently transformed into acoustic
oscillations or into radiated sound waves. The ideal transducer
system must, moreover, ensure that the electrical oscillations
applied are (within the audio frequency range) converted with such
amplitude and phase constants that square-wave oscillations within
a range of 100 to 3000 Hz are converted into appropriate sound
waves.
As far as practical application is concerned the relevant industry
is still a long way off this ideal condition (cf., for example,
"Hifi-stereophonie", Zeitschtift fur hochwertige Musikwiedergabe,
No. 5/69, Pages 362 to 340). Measurements taken on the most
sophisticated of known electro-acoustic transducers have clearly
shown not only that all applied electrical oscillations will
invariably result in a sound wave train composed of several waves
but also that (in dependence on the design and construction of the
transducer system) the sound will be reproduced as a distorted
sound wave.
The cause for these interferences has been traced to the fact that
conventional transducer systems have to be equipped with at least
one mechanical energy store. The energy retained in this store
during the pulse rise time is, in accordance with the time constant
of the store, either instantly or with a delay re-fed to the
subsequent cycle. The re-applied energy will thus not only result
in distortions but will also extend the radiated wave beyond the
limits of the oscillation applied.
The energy is stored in essentially two ways, one of which is the
force of inertia and the other by the force of elasticity. An
elastic energy store may be represented by the springs required to
restore the moving coil to its initial position or by the centering
diaphragms used for calotte loudspeakers. This acoustic compliance
will, on deflecting the moving coil from its rest position, result
in an amplitude-dependent restoring force which reduces the
amplitude of the applied oscillations and increases the amplitude
on restoring the moving coil. This in turn results in a distortion
of the applied oscillations on the one hand and will, on the other
hand, cause an overshoot of the moving coil beyond the rest
position with the after-oscillations resulting therefrom. The same
adverse influence can be attributed to those of the elastic type
energy stores especially produced by encapsulated air chambers in
small loudspeakers; in fact such elastic stores are occasionally
even produced for restoring the moving coils and thus take the
place of the restoring springs.
The loudspeaker diaphragm coupled to the moving coil of the
electro-acoustic transducer system may be regarded as an energy
store based on the force of inertia. Experiments have proved that
ideal functioning of the diaphragm can only be warranted if, during
deflection of the moving coil, each point of the diaphragm is
deflected with exact value. This condition could imaginably be met
by a material with as infinite rigidity (modulus of elasticity)
which material will be not elastically deformed even at maximum
frequencies. Such materials, of a necessarily small mass, are not
available. The force of inertia produces natural oscillations
inavoidably, therefore, in the known types of diaphragms, at least
within the higher frequency ranges.
Attempts have been made to attenuate the natural oscillations of
the diaphragm or to exclude their adverse effect on the radiated
sound waves. Among other methods attempts have been made to arrive
at the essential attenuation by using a woven or bonded fibre
fabric as diaphragm material or, alternatively, by impregnating it
with a suitable varnish; the diaphragm has also been constructed in
a way which assured that at a higher frequency certain sections of
the diaphragm, e.g. the marginal sections, take no active part in
the radiation of sound waves. Such measures, however, do not result
in a remedy of the adverse conditions but rather in an improvement
because they are only aimed at a diminishing of the effects of the
energy stores and not at an elimination of the stores themselves
formed by the diaphragm or the diaphragm construction. Thus, the
conventional electro-acoustic transducer systems suitable for all
sound frequency ranges comprise a number of transducers each of
which has a frequency range of its own (e.g. for treble, middle and
bass). Such a construction, therefore, offers the further
disadvantage that the rise times of the sound pressure edges in the
different transducers do not occur simultaneously but in a
sequence. This can be attributed to the fact that LC elements have
to be utilised for the determination of the different frequency
ranges and also to the use of different masses for the
diaphragms.
It has been the purpose and intention of this invention to create
the possibility of designing an electro-acoustic transducer which
eliminates the said disadvantages and which -- in particular -- is
operated without the said power stores.
According to the present invention there is provided an
electro-acoustic transducer system with a diaphragm coupled to a
moving coil, the diaphragm consisting of a flat textile carrier
element which is impregnated with a highly attenuating filling
material and which is made up of a material which is highly elastic
in its plane but substantially unelastic in bending.
There is also provided a diaphragm for the electro-acoustic
transducer system comprising a substantially flat textile element
being impregnated with an intensively attenuating filling material,
and having highly flexible properties in its plane but being
substantially unelastic in bending. The textile element is
preferably composed of a knitted fabric and is preferably fixed
with at least one point to a stationary part of the transducer
system. Specially suitable for this purpose is a knit or warp knit
material which can be extensively stretched in every direction.
This textile diaphragm element, may, within the scope of this
invention, be treated from both sides with a filling material. This
filling material should preferably be applied to a knitted or warp
knitted material under a little pre-tension.
There is also provided a method of producing the diaphragm and
mounting it in the electro-acoustic system comprising the steps of
centering the moving coil within the air gap; placing the textile
element on the moving coil ring and on the outer ring, tensioning
it radially from several sides preventing thereby creasing, coating
it with a diluted filling material within the range of the marginal
edges of the outer ring and the moving coil for the purpose of
local fixing, and drying it; and then applying the filling material
from at least one side to the whole textile element, pre-drying it
at a lower temperature and subsequently after-treating it at a
raised temperature.
With this invention it is for the first time possible to repudiate
the generally accepted opinion that diaphragms must be of a high
rigidity if they are, in their entirety i.e. as a rigid body, to
follow the reciprocating motions of the moving coil. This principle
had been universally applied to all known electro-acoustic
transducers or loudspeakers respectively. Measurements and tests,
however, have now proved the fallacy of this rule. It must rather
be ensured that all moving mass-points are, independently of their
amplitude, deflected with the same phase i.e. that the oscillation
of the diaphragm is carried out with perfect phase coincidence. It
has also proved to be of advantage if the amplitudes are not
identical at the different diaphragm points but rather decline in
their magnitude from a maximum at the coupling point to a minimum
value at the diaphragm edge. Such a phase coincidence and a decline
in the amplitude can be ideally realised with the diaphragm of this
invention. The most advantageous condition is established if the
diaphragm is not only fixed to the moving coil but also to a
stationary part of the transducer. This fixing method will warrant
that, commencing with the coupling point on the moving coil and
progressing towards the fixing point on the stationary part, the
oscillation amplitudes gradually decrease and also that a perfect
phase coincidence is also maintained within the range of the fixing
point. The easily extensible and highly attenuating material used
for the diaphragm of this invention will, moreover, ensure that the
diaphragm oscillations have no adverse effect on the motion
geometry of the moving coil or on the radiated sound waves.
Another decisive advantage offered by the diaphragms of this
invention is presented by the fact that they are suitable for
wide-band loudspeakers. Measurements taken have shown that these
diaphragms follow the oscillations within the bass range
practically in their entirety, the amplitudes of these oscillations
decreasing, however, as already described, in their magnitude to
the outside commencing from the point where they are coupled to the
moving coil, whereas a gradually decreasing section of the
diaphragm is set in oscillation with increasing frequencies until
at maximum frequencies only a minute section (a few millimeters
around the coupling point) of the diaphragm is energised. Thus it
is ensured that a desirably increasing radiation surface is made
automatically available as the frequencies become lower while, on
the other hand, the marginal zones being critical for the
excitation of natural oscillations are automatically excluded at
high frequencies. A special property of the diaphragm of this
invention is found in the fact that it may be regarded as an
oscillating system comprising a multitude of individual mass-points
interconnected by springs; the attenuation effect of the filling
material inhibits a high elasticity of these springs which thus are
provided with a considerable internal damping. These viscoelastic
properties have made it possible, as documented with tests
conducted with a torsional pendulum, that the logarithmic damping
decrement ascertained for the diaphragm of this invention exceeds
those of conventional diaphragms (of bonded fibre fabrics etc.) by
four to 15 times.
This invention has, for the first time, created the possibility of
designing and constructing a transducer system without making use
of a mechanical energy store. This system can be further developed
by providing a transducer without the detrimental enclosed air
chambers on the input or the output side of the diaphragm, and by
replacing the acoustic compliance used for restoring the moving
coil with a system of electro-magnetic centering (British Pat. No.
1286687, U.S. Pat. No. 3686446). An electro-acoustic transducer
system incorporating these features offers ideal properties,
because all mechanical energy stores are as small as possible and a
deceleration of the masses is not obtained by means of elastic
means as in known transducer systems, but by means of frictional
resistances. The energy released during the deceleration will,
therefore, be dissipated in the form of heat and in consequence can
not adversely influence the subsequent cycles of oscillation.
Embodiments of the present invention will now be described in
detail by way of reference to the accompany drawings, wherein:
FIG. 1 is an electro-acoustic transducer with a diaphragm mounted
in accordance with this invention,
FIG. 2 is an electro-acoustic transducer with enclosed air
chambers,
FIGS. 2a and 3 show electro-acoustic transducer systems without the
detrimental hollow spaces serving as energy stores,
FIG. 4 shows the oscillation parameters for different transducer
systems, and
FIGS. 5 and 6 show a knitted and a warp knitted fabric.
The electro-acoustic transducer systems shown in FIGS. 1 to 3 are
characterised in that the rest position of their moving coils and
consequently also of their diaphragms is determined by
electro-magnetic forces.
The design as shown in FIG. 1 provides for two permanent magnets
represented by pot magnets 8 and 9. The arrangement of these
magnets gives them the same axis of symmetry with their apertures
facing each other. Two moving coils 12, 13 rigidly interconnected
via bars 16, 17 or by a hollow cylinder or similar means fixed to a
diaphragm 14 are movable within annular air gaps L.sub.1, L.sub.2.
The diaphragm 14 consequently follows the rhythm of the reciprocal
motions of the moving coils 12, 13. The two bars 16, 17 may be
replaced, by a moving coil former 18. A known electro-magnetic
system (British Pat. No. 1286676, U.S. Pat. No. 3686446) is used
for the restoring of the moving coils 12, 13 and of the diaphragm
14 coupled to them.
The outer margin of the diaphragm 14 is fixed to a stationary outer
ring 19 which is fitted to a stationary part of the transducer
system (e.g. to the pot magnet 9) with a suitable bracket such as
non-magnetic webs 21. In its rest position the diaphragm 14 is
preferably arranged perpendicularly (or as near to perpendicularly
as possible) to the axis of the moving coils 12, 13.
The form of construction as shown in FIG. 2 provides for only one
pot magnet containing (as indicated in FIG. 2) two moving coils in
tandem arrangement. This pot magnet is assembled of a plurality of
pole shoe segments 23 and permanent magnet segments 24 arranged at
a given spacing on a circular arc. A diaphragm 26 is fixed to a
moving coil former 25 supporting the moving coils; the marginal
edge of this diaphragm is rigidly clamped down by an outer ring 27
which is fixed to the pot magnet 23. The centre of the circular
diaphragm 26 can also be retained in a fixed position with a pin
fitted to the pot magnet. These details are not shown on the
drawing. In its rest position the diaphragm is arranged in relation
to the moving coil axis.
The radiating surface of diaphragm 26 (FIG. 2) is not, as in a
conventional calotte loudspeaker, confined to the diaphragm section
fixed within the moving coil ring 26 but rather increased by the
entire diaphragm section stretched between the two rings 25 and 27.
Within the moving coil former 25, the diaphragm 26 is arranged in a
plane. Centering of the diaphragm 26 is accomplished by the known
means (British Pat. No. 1286687, U.S. Pat. No. 3686446).
On the back of the diaphragm 26, the inner and outer pole shoes of
the pot magnets 23 and the permanent magnet 24 form detrimental air
spaces 28 -- a phenomenon common with all conventional
loudspeakers. This can be avoided by designing the pot magnet, as
specified in FIG. 2a, to have two annular pole shoes 29, 30 and a
plurality of permanent magnets 31 arranged on an arc while leaving
spaces 36 free.
The embodiment according to FIG. 3 differs from that in FIG. 2a by
the fact that the air gap L is bordered on the one side by two
outer pole shoes 29a, 30a of permanent magnets 31 and on the other
side by a core 33 having the function of and being constructed as a
pole shoe ring. The core 33 is fixed to the pole shoes 29a, 30a via
webs of a non-magnetic material (details of which are not shown).
This offers a decisive advantage because there are no trapped air
spaces 28 (FIG. 2) at all which during the reciprocal movement of
diaphragm 26 could function as quasi-elastic energy stores and thus
adversely influence the oscillating behaviour of the diaphragm.
Another advantage of the designs according to FIGS. 1, 2a and 3 is
represented by the fact that sound waves are radiated to the front
as well as to the rear, because the transducer system is also
opened on the back of the diaphragm. If required a pin 35,
supported by thin webs 34, may be used for fixing stationary the
centre of diaphragm 26. In this case the diaphragm will be
absolutely symmetrically oscillated on both sides of the moving
coil ring 25. The arrangement of the diaphragm with the moving coil
ensures, that the diaphragm also in the deflected position as
indicated by the dotted line 26a in FIG. 3, is not stretched and
will consequently not form an elastic energy store even in this
position.
The diaphragm 14 or 26, consists of a viscoelastic material which
may be stretched in every direction. Particularly suitable for this
purposes is highly elastic knitware 14a (FIG. 5) as well as warp
knits 14b (FIG. 6). Textiles of this type offer the special
property that every single loop represents a thread reserve which
allows a continuous reforming of the thread position during a
stretching motion. A textile fabric of this type, therefore, can be
stretched to a degree unattainable by the rectilinear arranged
threads of a normal woven or bonded fibre fabrics. The knitted
fabrics offer on the other hand a high internal friction which in
turn ensures that only intensely attenuated natural vibrations can
be performed. The elasticity and attenuation property of this
material can be further increased by knitting the loops not of
single fibres but of a multifilament fibre with a thin single
capillary arrangement. Also suitable are goods knitted with stretch
yarns of S- and/or Z-twist, the yarns being produced by the
Helanca-method or by the false-twist-method and consisting e.g. of
three single filaments. Usable as fibre material are materials of
polyamide and polyester i.e. materials which in themselves offer a
certain elasticity. The use of inextensible fibres, such as glass
fibres, is by no means excluded because the essential stretching is
always ensured by the loops. The best possible results are achieved
with a fibre material which requires only little attenuating
material and thus provides for diaphragms of an extra light weight.
The use of too strongly crimped yarns should, therefore, be
avoided. A weight-ratio textile carrier to filling material of 1:5
is preferred.
The diaphragm material produced on a circular or flat knitting
machine, on a Cotton machine or on a warp knitting machine should
be coated with an attenuating material in accordance with the same
method as applied to diaphragm of woven or bonded fabrics. Suitable
attenuating materials are especially solutions such as solution of
butadiene copolymer (e.g. Butofan 380 D of the firm of BASF or
similar).
Several methods may be used for the construction and installation
of the diaphragm. The method for simultaneous construction and
installation as described with reference to FIGS. 2a should be
given preference.
The moving coil ring 25, wrapped by the moving coils, is at first
centered with the aid of suitable auxiliary fixtures within the air
gap of the pot magnet and then positioned in a way that ensures
that its front end is mounted flush with the outer ring 27. A
textile carrier element, such as a section cut from a fine lady's
stocking, is then placed on the two rings 25 and 27 with their axes
preferably in a vertical plane. Small weights, e.g. of eith grams
each, are then suspended from the projecting edge of the knitted
fabric at a uniform spacing. The textile carrier element is thus
smoothly stretched at a low pre-tension. During this step the
knitted fabric stretched over the rings 25, 27 should not form
creases and the loops should be straight and parallel to each
other. The leads of the moving coil can be drawn through the
knitware from back to front if required.
After having precisely centered the moving coil and uniformly
stretched the knitware in the prescribed manner, the knitware
within the range of the marginal edges of the outer ring or of the
moving coil, respectively, is coated with a dilute solution of the
filling material which is then air dried to fix the fabric on the
rings 25 and 27. Suitable for this prupose is, for instance, a 20
percent aqueous solution of Butofan 380 D.
The duly fixed fabric is then coated from the front with a 50
percent Butofan solution which is pre-dried at an air temperature
of 70.degree. C. The back of the fabric is then treated in a
similar manner with filling material. The coating should generally
just be thick enough that no fibre end will protrude through it on
either side of the fabric. A thicker layer will be applied on both
sides within the vicinity of the two rings 25, 27. Subsequently the
diaphragm simultaneously produced and fixed to the rings 25 and 27
will be dried at a high temperature e.g. at 130.degree. to
150.degree.C. The diaphragm which has been stretched to a certain
limit during the pre-drying process now relaxes almost completely
due to the treatment at an elevated temperature and forms creases,
which permits for the moving coils a reciprocal movement to the
extent of ten to twenty millimeters in both directions without
stretching the diaphragm thereby in a radial direction. The layer
of filling material applied to the highly elastic knitware has
converted this material into a textile element which can still be
stretched in every direction but in comparison with the original
knitware, not only has an essentially increased modulus of
elasticity but also visoelastic behaviour.
The weights are removed after completion of the drying process, the
diaphragm edges protruding beyond the outer ring 27 are trimmed and
the auxiliary fixture dismantled.
The simultaneous construction and fitting of the diaphragm offers
the decisive advantage that after this treatment the moving coil is
centered in a direction perpendicular to the direction of motion
alone by the diaphragm and is thus ready for use without any
further measures being necessary.
Another possible method of making and fixing the diaphragm
comprises first pre-fixing the diaphragm material at 195.degree.C
so that it has 140 loops per 49 mm.sup.2 (square millimeters).
After pre-loading by the weights, the diaphragm material is then
clamped to the outer ring and separated, whereupon the coil is
arranged concentrically of the outer ring by means of an auxiliary
fixture. The whole arrangement is finally passed under a curtain of
filling material, for example Butofan 390 D, it being possible to
provide pre-drying and final drying as in the above example. Fixing
of the diaphragm to the outer ring, fixing of the coil to the
diaphragm and appropriate filling of the diaphragm material are
carried out in one process step.
A diaphragm of a plain knitted fabric, produced on a circular
knitting machine of a 20/3/1 Nylon-Yarn, was used for an exemplary
embodiment of the invention; the sequential loop course of this
fabric are composed of yarns alternatingly S-twisted and X-twisted,
with a twist of 1800 to 2400 revolutions per meter of yarn. The
fabric was subsequently drawn over a cylindrical former in a manner
which ensured that the individual loops were twisted out of the
knitting plane until they were uniformly and vertically aligned in
relation to this plane. In this condition the fabric was pre-fixed
at a temperature of about 105.degree. to 110.degree.C and
simultaneously dyed. After completion of these process steps the
fabric was once more drawn over a cylindrical former until a loop
density of about 3 loops per square millimeter was attained. In
this condition the diaphragm material was once more fixed. Prior to
the treatment with the filling material the loops should preferably
protrude by 0.2 to 0.3 mm from the knitted fabric in order to
provide a diaphragm thickness of approximately 0.25 to 0.4 mm after
treatment with the filling material.
The finished diaphragm offers, according to DIN 53362, a bending
resistance of 1 to 10 g/cm.sup.2 and a specific gravity of 1 to 1.1
g/cm.sup.3. The logarithmic attenuation decrement is 4 to 20 if
exposed to a torsional oscillating test according to DIN 53445 or
ISO/DOR 533, respectively. The tensile test shows that with a pure
carrier material (knitted fabric) an elongated of 10 percent is
achieved with a weight of 2.5 g within a few minutes, while in case
of the finished diaphragm the same elongation is reached with
weight of 260g, the process becoming stable only after a period of
30 min. During operation of the electro-acoustic transducer system
as described in FIGS. 1 to 3 the moving coils are deflected in an
axial direction depending on the electrical oscillation applied;
the diaphragms fixed to the moving coils and possibly clamped by
their edges being thus set in reciprocating motion. Within the bass
range, i.e. at relatively large deflection amplitudes, the
diaphragms will move in their entirety because the tensile forces
acting on the points at which the diaphragms are fixed to the
moving coil formers 18 or 25 remain in-phase effectively up to the
diaphragm edge or the diaphragm centre (FIG. 3). An ever smaller
part of the diaphragm participates in the reciprocating movements
as the frequencies increase and the deflection amplitudes decrease
until only a very minor part of the diaphragm is set in oscillation
and acts as radiating surface at very high frequencies. In this
case the oscillating parts extend barely a few millimeters from the
fixing point in a radial direction. Identical conditions will be
produced on both sides of the moving coil formers 18, 25 depending
on whether or not the diaphragm is clamped on only one side (FIGS.
1, 2 and 2a) or on both sides (FIG. 3). Although the diaphragm may
be fastened to the moving coil ring 25 in a manner which divides it
into approximately two equal sections, of which one is arranged
outside and the other inside the moving coil ring, in the preferred
arrangement the diameter of the moving coil former 25 is equal to
the radius of the diaphragm 26 or of the outer ring 27. This
ensures that the diaphragm is held by the moving coil former 25
precisely centrally.
The diameter of the moving coil and of the coil former should be
adequate to ensure, in conjunction with electro-magnetic-centering
a non-tilting positioning of the coil former i.e. so that its axis
is kept parallel to that of the air gap. The consequence of a
moving coil former with an inadequate diameter could be that even a
slight asymmetry in fixing the diaphragm would result in tilting of
the moving coil such that it would come to rest on the pole shoe
surfaces. Diameters of 50 to 90 mm for example are preferred for
the moving coil former.
The pole shoes should offer surfaces of approximately the same size
on both sides of the moving coil former 25 in order to ensure that
the air cushions behind the diaphragm, which are to be displaced on
return diaphragm motions, are symmetrically arranged.
The transducer system according to this invention offers decisive
advantages over conventional calotte loudspeakers in that the
radiating surface of the diaphragm can be drastically increased
without adverse natural oscillations being generated. This
advantageous feature can be realized because the diaphragm material
has no adverse influence on the oscillatory behaviour of the
transducer system and because that part of the diaphragm, which
radiates sound waves at a certain frequency, is automatically
adjusted within the diaphragm in accordance with the power
transmission ratio.
The invention is not limited to the examples given but can be
modified in several ways. The magnet designs, as shown in FIG. 3,
may, for instance, be replaced by other designs without any
enclosed air chambers on the back of the diaphragm.
The diaphragm of this invention is by no means limited to the
design examples described even though the transducer system
according to FIG. 2a offers a maximum of advantageous features as
its diaphragm and the electro-magnetic restoring excludes the three
detrimental energy stores. The diaphragm of this invention may also
be used with calotte loudspeakers. Because the diaphragm of this
invention, however, has only a low rigidity it will be within the
former 25 of a flat type according to FIGS. 2, 2a and not of the
usual calotte configuration. The radiating wedge can be enlarged if
the diaphragm is fixed at its centre according to FIG. 3. The
invention is, moreover, not limited to transducer systems with
electro-magnetic restoration but can also be used with the
conventional transducer systems making use of a mechanical
restoration by using for the diaphragms, as specified on FIGS. 1 to
3, additional centering diaphragms or restoring springs.
A wide scope is also given to the shape, material and arrangement
of the diaphragms. Preference is given to circular or square
diaphragms configurations with a symmetrical coupling to the moving
coil but without excluding the use of other diaphragm
configurations. Highly suitable for these diaphragms are knitted
fabrics made on flat or circular knitting machines; no
specifications are offered as to weaves and loop sizes, which must,
however, ensure that high flexibility is maintained. Highly
suitable are plain or rib knitted fabrics such as can be cut from a
standard fine gauge lady's stocking. The arrangement given to the
diaphragms on FIGS. 1 to 3 ensures that the diaphragms in their
rest position are located vertically to the axis of motion of the
moving coils. Modifications are possible and the drive axis as well
as the diaphragm surface may be arranged at an angle other than
90.degree. as a modified angle will possibly influence the size and
position of the radiating wedge but never the double amplitude and
phase coincidence transducing effect. The oscillating behaviour of
an ideal, a conventional and of an electro-acoustic transducer
system of this invention, are shown diagrammatically in FIG. 4.
FIGS. 4a and b show each a sine and a square oscillation applied to
the transducer, the amplitudes being drawn alongside the ordinate
while the time axis is represented by the abscissa. The ideal
configuration of acoustic oscillations as emitted by the transducer
are represented in FIGS. 4c and 4d. FIGS. 4e, 4f, 4g and 4h show
that the sound oscillations as produced by a conventional
transducer are, within the period of the applied sine wave, not
only strongly distorted by the interfering energy store but also
gradually declining along a branch beyond the end of the period.
The transducer, as specified in this invention, produces according
to FIGS. 4i and j within the entire sound frequency range curves
which can be hardly distinguished from the ideal ones shown in
FIGS. 4c and d.
The advantages offered by the transducer system as specified in
this invention may not only be applied to loudspeakers and
diaphragms but also to microphones and headphones.
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